CHAPTER 6 – DEVELOPING A PROJECT PLAN
OBJECTIVES
To establish the linkage between the WBS and the project network To diagram a project network using AON methods To provide a process for computing early, late and slack activity times and identify the critical path To demonstrate understanding and application of lags in compressing projects or constraining the start or finish of an activity To provide an overview framework for estimating times and costs To suggest the importance of slack in scheduling projects
DEVELOPING THE PROJECT NETWORK
The Project Network
A flow chart that graphically depicts the sequence, interdependencies, and start and finish times of the project job plan of activities that is the critical path through the network.
Provides the basis for scheduling labor and equipment. Enhances communication among project participants. Provides an estimate of the project’s duration. Provides a basis for budgeting cash flow. Identifies activities that are critical. Highlights activities that are “critical” and can not be delayed. Help managers get and stay on plan.
FROM WORK PACKAGE TO NETWORK
WBS/Work Packages to Network
FROM WORK PACKAGE TO NETWORK (contd.)
WBS/Work Packages to Network (contd.)
CONSTRUCTING A PROJECT NETWORK Terminology
Activity: an element of the project that requires time. Merge Activity: an activity that has two or more preceding activities on which it depends. Parallel (Concurrent)
A
CONSTRUCTING A PROJECT NETWORK (contd.)
Terminology
Path: a sequence of connected, dependent activities. Critical path: the longest path through the activity network that allows for the completion of all project-related activities; the shortest expected time in which the entire project can be completed. Delays on the critical path will delay completion of the entire project. C
B
D
C
A
B
D
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CONSTRUCTING A PROJECT NETWORK (contd.) Terminology
Event: a point in time when an activity is started or completed. It does not consume time. Burst Activity: an activity that has more than one activity immediately following it (more than one dependency arrow flowing from it).
Basic Rules to Follow in Developing Project Networks
Networks typically flow from left to right. An activity cannot begin until all of its activities are complete. Arrows indicate precedence and flow and can cross over each other. Identify each activity with a unique number; this number must be greater than its predecessors. Looping is not allowed. Conditional statements are not allowed. Use common start and stop nodes.
Two Approaches
Activity-on-Node (AON)
Uses a node to depict an activity.
B
Activity-on-Arrow (AOA)
Uses an arrow to depict an activity.
A
C
D
Network Computation Process
Forward Pass—Earliest Times
How soon can the activity start? (early start—ES) How soon can the activity finish? (early finish—EF) How soon can the project finish? (expected time— ET)
Forward Pass Computation
Add activity times along each path in the network (ES + Duration = EF). Carry the early finish (EF) to the next activity where it becomes its early start (ES) unless… The next succeeding activity is a merge activity, in which case the largest EF of all preceding activities is selected.
Backward Pass—Latest Times
How late can the activity start? (late start—LS) How late can the activity finish? (late finish—LF) Which activities represent the critical path? How long can it be delayed? (slack or float—SL)
Backward Pass Computation
Subtract activity times along each path in the network (LF - Duration = LS). Carry the late start (LS) to the next activity where it becomes its late finish (LF) unless The next succeeding activity is a burst activity, in which case the smallest LF of all preceding activities is selected.
Determining Slack (or Float)
Slack (or Float)
The amount of time an activity can be delayed after the start of a longer parallel activity or activities.
Total slack
The amount of time an activity can be delayed without delaying the entire project.
The critical path is the network path(s) that has (have) the least slack in common.
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Practical Considerations
Network Logic Errors Activity Numbering Use of Computers to Develop Networks Calendar Dates Multiple Starts and Multiple Projects
Extended Network Techniques to Come Close to Reality
Laddering
Activities are broken into segments so the following activity can begin sooner and not delay the work.
Lags
The minimum amount of time a dependent activity must be delayed to begin or end.
Lengthy activities are broken down to reduce the delay in the start of successor activities. Lags can be used to constrain finish-to-start, start-to-start, finish-to-finish, start-to-finish, or combination relationships.
Extended Network Techniques to Come Close to Reality (contd.) Hammock Activity
An activity that spans over a segment of a project. Duration of hammock activities is determined after the network plan is drawn. Hammock activities are used to aggregate sections of the project to facilitate getting the right amount of detail for specific sections of a project.
Key Terms
Activity Activity-on-arrow (AOA) Activity-on-node (AON) Burst activity Concurrent engineering Critical path Early and late times Gantt chart Hammock activity Lag relationship Merge activity Network sensitivity Parallel activity Slack/float—total and free
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